1. Field of the Invention
The present invention relates to a laser machining apparatus for cutting or boring a workpiece by a laser beam, and particularly to a laser machining apparatus suited for piercing a metal plate workpiece.
2. Description of Related Art
In general, laser machining is performed by irradiating a converged laser beam on a workpiece through a small hole of approximately 0.5 mm to 5 mm in diameter and also by jetting assist gas of oxygen, air, nitrogen or argon through the small hole, to perform cutting, welding, boring, etc. Also in piercing a workpiece, assist gas is generally jetted coaxially with the optical axis of a laser beam. Particularly in piercing a metal workpiece, oxygen is used as an assist gas to cause oxidization on a workpiece to facilitate piercing by utilizing a phenomenon of heating and melting associated with combustion.
Conventionally, in piercing a steel plate and the like by a laser beam, a laser beam is outputted intermittently as pulses in order to suppress melted metal flying up over a workpiece. However, the larger the plate thickness of a workpiece is, the longer time it takes to pierce the workpiece. In order to make the time required for piercing shorter, the power of a laser beam is increased and piercing is performed by outputting a laser beam continuously instead of outputting as pulses. In such manners of piercing, a large amount of spatters and slugs are formed by melted metal and metal oxide flying up over a workpiece. (Here, slugs mean lumps of melted or melted and solidified metal and metal oxide, and spatters mean particles thereof.)
Spatters and slugs adhering to the surface of a workpiece hinder cutting and the like and lower the machining precision. In order to deal with those problems, there has been proposed a method of piercing in which a jet of blow gas of air, nitrogen or the like is directed obliquely from an upper side to a machining portion to blow off produced spatters and slugs in Japanese Patent Laid-Open Publication Nos. 9-277071 and 10-225787.
In the method of piercing by jetting blow gas obliquely from an upper side to a machining portion, the blow gas is jetted transversely with respect to an optical axis of the laser beam and thus an axis of the assist gas jet. Therefore, there is a possibility that the blow gas may hinder the assist gas being supplied to the machining portion. Thus, the supply pressure of blow gas needs to be controlled in accordance with supply pressure of the assist gas.
Even if spatters and slugs can be removed effectively by controlling the supply pressure of the blow gas in accordance with the supply pressure of the assist gas optimally, there is another problem in the case where the workpiece has a large thickness. The larger thickness the workpiece has, the longer time it takes to pierce the workpiece, and therefore a pierced hole suffers deformation in the blow direction. Further, another problem may be that all the spatters and slugs cannot be removed from the workpiece because the pressure of blow gas cannot be raised sufficiently.